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14°

PRACTICAL ORGANIC CHEMISTRY

PREPARATION 46.
Bromobenzeiie (Phenyl bromide), CcH6Br.
Cohen and Dakin, Trans. Chem. Soc., 1899, 76, 894; Cross
and Cohen, Proc. Chem. Soc., 1908.
50 grms. benzene.
120     „      (40 c.c.) bromine,
o* 5  „       pyridine.
The apparatus is similar to that shown in Fig. 63, p. 89, but tine
flask should be placed in a water-bath, in which it can be heated.,
and the tap-funnel may be dispensed with. The benzene, bro-
mine, and pyridine are placed in the flask and heated to 25—30°,
when a vigorous and steady evolution of hydrogen bromide
takes place, the gas being absorbed by the water in the beaker.
When the action slackens (about i hour) the temperature of tlie
water-bath is gradually raised to 65—70°, and the process
stopped when most of the bromine has disappeared and trie
evolution of hydrogen bromide has nearly ceased. The con-
tents of the flask are cooled and poured into dilute caustic soda,
solution contained in a separating funnel and shaken. Suffi-
cient alkali must be present to give an alkaline reaction after
shaking. The lower layer is drawn off and dehydrated over
calcium chloride. When perfectly clear the bromobenzene is
filtered or decanted into a distilling flask (200 c.c.) provided
with a thermometer and distilled over wire-gauze. Unchanged
benzene first passes over ; the temperature then rises rapidly
and the portion boiling at 140—170° is collected separately. It
is redistilled and collected at 150—160°. Yield 60 grams.
C0HC -1- Br2 = C0H5Br + HBr.
The pyridine acts as "halogen carrier," probably by forming"
the additive compound C5H5NBr2, which gives up its bromine to
the benzene.
Properties.—Colourless liquid ; b. p. 154—155° ; sp. gr. i"49<5
at 16°.
Hydrobromic Acid.—The weak solution of hydrobrom i c
acid which collects in the beaker in the course of the above re-